Contrary to all expectations, the mysterious dark energy that is pushing the Universe apart may be changing with time.

By observing distant, powerful bursts of gamma rays (-rays), Brad Schaefer says he has preliminary evidence that the strength of dark energy is different today from when the Universe was very young. Schaefer, an astronomer at Louisiana State University in Baton Rouge, presented his results at an American Astronomical Society meeting in Washington DC.

Just minutes after the data were presented in a late afternoon session, some astronomers were already calling the bold claim into question.

An idea that arose in the late 90s, dark energy seems to act over very large distances, pushing the Universe apart at an ever increasing speed. At the moment, many researchers believe that dark energy may be a foam of quantum particles that exists throughout the vacuum of space. Under that scenario, dark energy would be a constant and unchanging force, according to Michael Turner, a cosmologist from the University of Chicago, Illinois.

Schaefer's findings, if they are true, would turn that idea on its head.

Big, bright bursts

Schaefer began by examining 52 bursts of high-energy -rays that were recorded by six satellites. The bursts came from the explosions of distant, super-massive stars that were billions of light years away. The explosions were so distant that the ones we see now occurred when the Universe was only 1.5 billion years old, a tenth of its current age.

Schaefer used various properties of the bursts, such as the way their brightness changed over time, to work out how intrinsically bright they were and how far away the explosion was. Twelve of the most distant of the bursts examined by Schaefer turned out to be brighter than would be expected if dark energy had remained a constant.

"I would not characterize this as proof," Schaefer cautions. "Before we can be confident, we will have to see these results reproduced."

The old ways

Many questions surround Schaefer's result, says Dieter Hartmann, a -ray astronomer at Clemson University in South Carolina.

For one, says Hartmann, the findings are dependent on understanding just how the ancient, massive stars lived and died. "The births of these stars will be different, and the environment in which they ultimately explode is different," he says. Without understanding the physics of these explosions, the estimate of their intrinsic brightness is very uncertain, he says.

Others are still more sceptical. "I think he's wrong," says Adam Riess, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland. Riess says that Schaefer's failure to include nearby and recent -ray bursts to calibrate the results make his conclusions about the much more distant bursts highly questionable.

"This is an intriguing result," admits Turner. But it certainly doesn't add up to a Nobel prize without further confirmation, he says. "I don't think it's a ticket to Sweden."